The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Hydrocarbons (oil, condensate, and gas) are typically produced from wells that are drilled into the formations containing them. For a variety of reasons, such as inherently low permeability of the reservoirs or damage to the formation caused by drilling and completion of the well, the flow of hydrocarbons into the well is undesirably low. In this case, the well is “stimulated” for example using hydraulic fracturing, chemical (usually acid) stimulation, or a combination of the two (called acid fracturing or fracture acidizing).
Hydraulic Fracturing is a stimulation process commonly used in order to enhance hydrocarbon (oil and gas) productivity from the earth formations where these resources are accumulated. During hydraulic fracturing, a fluid is pumped at rates and pressures that cause the downhole rock to fracture. Typical stages of a fracturing treatment are the fracture initiation, fracture propagation and fracture closure. During fracture initiation fluids are pumped into a wellbore connected to the formation through entry points such as slots, or perforations, to create a typically biplanar fracture in the rock formation. During propagation, fluids are pumped to grow the fracture primarily in the longitudinal and vertical direction, for which fluids are pumped into the wellbore at rates exceeding the rate of fluid filtration into the formation, or fluid loss rate. Optimal fracturing fluids pumped to propagate fractures typically have rheological characteristics that promote a reduction of the fluid loss rate, and serve the purpose of maintaining a certain width of the created fracture at the rate and pressure at which the fluid is pumped downhole, what in return increases the efficiency of the treatment, defined as the volume of fracture created divided by the volume of fluid pumped. Upon cessation of flow, the downhole formation tends to close the fracture forcing the fluid in the fracture to further filtrate into the formation, and or into the wellbore.
In some treatments, know as acid fracturing treatments, in order to maintain some connectivity between the created fracture and the wellbore, acids are incorporated into the fluid (dissolved, or suspended) which are capable of etching some of the minerals in the formation faces, thus creating areas of misalignment through which hydrocarbons can flow into the wellbore from the formation.
In other treatments, known as propped fracturing treatments, solid particulates of sizes substantially bigger than the grains in the formation known as proppant, which are capable of substantially withstanding the closure stress, are pumped with the fluid in order to prevent complete fracture closure (prop the fracture open) and to create a conductive path for the hydrocarbons.
A few different methods of creating propped hydraulic fractures are known. Many treatments requiring a substantial width formation resort to the use of viscous fluids capable of reducing fluid loss, typically aqueous polymer or surfactant solutions, foams, gelled oils, and similar viscous liquids to initiate and propagate the fracture, and to transport the solids into the fracture. In these treatments the fluid flow rate is maintained at a relatively high pump rate, in order to continuously propagate the fracture and maintain the fracture width. A first fluid, known as pad, is pumped to initiate the fracture, which is pushed deeper into the reservoir by propagating the fracture, by the fluid pumped at later stages, known as slurry, which typically contains and transports the proppant particles. In general the viscosity of pad and slurry are similar, facilitating the homogeneous displacement of the pad fluid, without substantial fingering of one fluid into the other.
Recently a different method of creating propped fractures has been proposed in which a viscous fluid and a slurry fluid are alternated at a very high frequency, allowing for heterogeneous placement of proppant in the formation.
Another method of creating propped fractures very common in low permeability reservoirs where fluid viscosity is not typically required to reduce fluid loss is the use of high rate water fracs or slick water fracs. In these treatments, the low viscosity slurry is typically not able to substantially suspend the proppant, which sinks to the bottom of the fracture, and the treatment relies on the turbulent nature of the flow of a low viscosity fluid pumping at a very high velocity above the proppant to push the proppant deeper into the formation in a process called dunning, (because is similar to the dune formation in sandy areas, where the wind fluidizes the sand grains on the surface, and transports it for a short distance until they drop by gravity), creating a front that smoothly advances deeper and deeper into the fracture. In this case, proppant slugs are pumped, at very low proppant concentrations to prevent near wellbore deposition (screenout) followed by clean fluid slugs aiming to push the sand away from the wellbore.
Hybrid treatments where fractures are opened with one type of the fluids and propped with a different fluid can be envisioned and are also known, and practiced in the industry.
Matrix treatments are stimulation treatments in which a fluid capable of dissolving certain components naturally occurring in the formation, or deposited near the wellbore during drilling, cementing, or production is pumped into the formation at a rate and pressure substantially smaller that those required to initiate a fracture in the formation. Matrix treatments are typically pumped into formations in order to reduce the skin around the wellbore, restoring the natural conductivity of the formation, which is typically damaged by the drilling and cementing fluids that are used to complete the wellbore. Acids, and solvents, are typically pumped for this purpose. Generally solids are not pumped in these matrix treatments with the purpose of transporting them deep into the reservoir, since they would typically not travel far into the formation, due to the tortuous porous path resulting from these dissolving treatments. Instead, solids can be pumped in matrix treatments in order to divert near wellbore, the flow of fluid from given zones of the reservoir towards others.
It is a purpose to disclose a new method of propping at matrix rate through a prior fracture stimulation treatment.